The invention relates to a wear indicator system for metal pipes which are part of an offshore structure or a pipeline and are covered with a corrosion protection system.
Offshore structure designates stationary structures which are erected in the open sea off the coast. Examples of such offshore structures are wind power plants, drilling rigs and lighthouses. The pipelines in question are rigid pipes having a solid metal wall, as opposed to flexible pipes of multilayer construction.
The metal pipes are, for example, part of the foundation structure of offshore structures. The foundation structure of offshore structures is the area which carries the actual functional unit. In the case of wind power plants, the foundation structure carries the tower including turbine and rotors. In the case of drilling rigs, the foundation structure carries the platform including superstructures. In the case of lighthouses, the foundation structure carries the tower, if present, and the light. The foundation structure lies underwater, in the intertidal zone, in the foam zone and possibly in the aerosol zone. The foundation structure includes the foundation elements with which it is anchored in the sea floor.
In the course of the planned expansion of wind power utilization, a large number of offshore wind power plants are planned for the coming years both in the North Sea and in other seas and in inland seas. The overall mechanical system of an offshore wind power plant comprises the components turbine, rotors, tower and foundation structure.
In order to base these plants on the bottom of the body of water, at locations up to over 100 km from the coast, specific constructions are required, which differ highly from the constructions in the onshore region. Some regions of these complicated constructions, such as monopiles, jackets, tripods, tripiles, etc., are subjected to high static and primarily dynamic and corrosive loading. Depending on the position and water depth at the respectively considered location, what are known as the 50-year wave and the tidal range have to be taken into account. Added to this are high UV radiation, salty spray, foam, aerosols, temperature changes, mechanical loadings, growth with mussels and other life forms and associated mechanical wear by animals and chemical ablation as a result of discharges from animals and other marine life forms. For these constructions, use is made of steel pipes which, for corrosion protection reasons, can be sealed off in an airtight manner or filled with concrete. Furthermore, power or other supply lines can run through the steel construction pipes.
To date, the metal pipes needed for the construction have been designed with substantially thicker wall thicknesses (up to 25%) than directly necessary and conventional paints, mostly based on epoxy resin or polyurethane, are used for the corrosion protection. As a rule, these paint systems do not offer particular protection against mechanical loading. In addition, these frequently have to be applied by hand at great heights, which results in quality control being difficult. The quality of such a coating is not comparable with the coating performed at the factory.
EP 2 511 430 describes that steel construction pipes which are covered with an extruded layer made of a polyamide moulding compound can be used in the foundation structure of offshore structures. Better protection against mechanical loadings and against corrosion and UV irradiation is achieved therewith than in the case of the previously known applicable pipes. The covering can also be formed in multiple layers.
Pipelines made of metal are currently frequently covered with a polyolefin such as polyethylene or polypropylene (WO 2002/094922; US 2002/0066491; EP-A-0 346 101). The coatings or coverings are used primarily for corrosion protection; they are described by corresponding standards. For the polyolefin covering, these are, for example, DIN EN 10288 and DIN 30678. In the case of the polyolefin covering, this layer is produced, for example, by means of tubular or wrapping extrusion. For the purpose of adhesion promotion, epoxy and adhesive layers can be applied one after another before the extrusion.
Conventionally, as regulated by DIN EN 10310 (German version EN 10310:2003), steel pipes for underground and water-laid pipelines are coated by means of polyamide powder. The polyamide coating is applied by dipping in a fluid bed, spraying on or in the roll application process. Because of the process, only relatively thin layers can be applied to the metal by means of powder coating. Disadvantageous in particular is the fact that a powder made of a relatively low-molecular weight polyamide has to be used for the coating, in order to ensure good flow of the melt on the hot metal surface. A coating obtained in this way is primarily used for corrosion protection. Furthermore, thermosetting coatings based on epoxy or polyurethane are also known.
In pipeline construction, higher technical requirements are to an increasing extent being placed on the pipe coating, since the environmental, laying and operating conditions are becoming more and more demanding. One of the most effective methods to protect underground pipelines against corrosion, in particular in the case of cathodic corrosion protection, is a multilayer covering. This consists of an epoxy resin layer as a first layer, a copolymer as adhesive as a second layer and an outer polyolefin layer made of polyethylene or polypropylene. This covering method can be applied to pipes from small to large. However, in the offshore and onshore area, high requirements are often additionally placed on the resistance against mechanical stresses. In order to take this problem into account as well, WO2010/094528 recommends the use of a metallic conduit pipe which is covered with an extruded layer made of a polyamide moulding compound, to produce a pipeline laid in water.
Damage to such corrosion protection coatings leads to the corrosion of the steel construction to be protected and can thus lead to structural endangering of the structure or the pipeline. Conventional procedures, therefore, firstly, primarily in offshore structures, include regular on-site inspection of the corrosion protection; secondly, the steel construction is designed with the inclusion of a safety factor which permits time-limited corrosion of the steel construction caused by damaged protective layers.
Since, for example, offshore wind power plants are not continuously occupied, the inspection of these wind power plants is associated with considerably increased expenditure; a visit to the plant is always necessary. As a result of external influences of the weather and of the sea, a visit or visual inspection of the plant is not always possible, however, which additionally makes the inspection more difficult.
The object of the invention is, therefore, to develop a wear indicator system for corrosion protection covering systems which, even in the case of non-regular occupancy of offshore structures, permits reliable assessment of the corrosion protection. In particular, travelling to these structures should therefore be limited to a minimum and, in addition, the safety margin in designing these structures can be reduced.